What should be done when a new question with a title like this ("feature problem" or "feature problem in Language" or "adjective feature problem in Language") appears? Is it a symptom of questions that are too localized?

It's certainly not ideal, but it's not the end of the world, either. There's not much we can do besides let the community sort it out through the usual editing process. Most of those questions are from before 2010, when there were fewer users with edit power. This answer is partially born of necessity, as I'm not aware of any vagueness detection algorithm that won't turn out loads of false positives and false negatives.

Probably the best we could do is a small warning like the "your question appears to be subjective" banner, but how many of those banners are we okay with having? Or, if modifying it rather than adding a new banner, how many purposes are we okay with having it serve? One seems like a good cap to me.

Yes, those questions area bit localized, but — I assume, since I haven't read each one — they do discuss different issues with regexes, and there's still some value to having them around. They certainly will be hard to find by people with similar problems, though.